1. Field of the Invention
The invention relates to communication systems and more particularly to a bit error probability (BEP) estimation method and a receiver employing the same.
2. Description of the Related Art
In radio communication systems, wireless links between base stations and their service terminals vary greatly and have a significant affect on system performance. Therefore, various technologies have emerged to handle different operating standards, such as Global System for Mobile telecommunication (GSM), General Packet Radio Service (GPRS) and the Enhanced Data rates for GSM (or Global) Evolution (EDGE) standards.
GPRS and EDGE include multiple coding/puncturing schemes and multiple modulation formats, such as Gaussian Minimum Shift Keying (GMSK) modulation or 8 Phase shift Keying (8PSK) modulation. Link adaptation (LA) is a mechanism used to adapt the channel coding and modulation schemes to the radio link conditions. LA allows the network to command the mobile set to change its modulation and coding/puncturing schemes according to the current radio condition, thereby providing qualified communication and high spectra efficiency. The actual bit error probability (BEP), changing with the variant channel, provides an accurate and timely measure representative of the change channel environment to facilitate LA. The evaluation of BEP is carried on either in the mobile set or the base station, and the estimation result is then sent back to the network for determination of the modulation format and coding/puncturing scheme by the LA mechanism.
FIGS. 1a and 1b respectively show data structure and formation procedure of a coded-block for data transmission in a transmitter of EDGE system. Referring to FIG. 1a, information bits are transmitted in a data packet DADAP in step 100 before entering the transmitter. FIG. 1b shows the data structure of the data packet DADAP. As shown, the data packet DADAP consists of a header part HP and one or two possible data parts DP. The header part HP, consisting of a header field HF and an uplink state flag (USF) field USFF, includes control and routing information associated with the data packet DADAP, for example, destination address of the data packet DADAP, error checking information, and control bits enabling receipt of the data packet DADAP to be acknowledged. The data part DP, the number of which depends on MCS type demonstrated in the header part HP, includes the information portion of the data packet DADAP. Referring to FIG. 1a, in the transmitter, USF field USFF, the header field HF and the data part DP respectively undergo steps 110, 112 and 114 to be encoded into a coded block CB conventionally referred to as a radio link control/media access control (RLC/MAC) block for data transmission.
In step 110, the USF field USFF of the header part HP is coded with a pre-coding procedure 110 to provide a coded USF field CUF. In steps 112 and 114, respectively including coding and puncturing steps 1121 and 1122, and 1141-1142, the header field HF and the data part DP are encoded and punctured to generate a coded header field CHF and a coded data part CDP respectively in order to increase reliability of the data transmission. The coded header field CHF and the coded USF field CUF are referred to as a coded header part CHP.
More specifically, in the coding steps 1121 and 1141, error correction coding or error detection coding, or both, for increased link reliability, is performed on the header field HF and the data part respectively. The coding may comprise, for example, cyclic redundancy check (CRC), convolution coding, Turbo coding, Trellis coding, block coding, or a combination thereof. As shown in FIG. 1a, the header field HF initially encoded with an outer code, e.g. particular CRC code (i.e. header check sequence) such that the CRC bits are appended to the header field HF is in sub-step 11211, and then the appended header field is tailed bitten in sub-step 11212, and the tailed-bitten header field is further encoded with an inner code, e.g. convolution code in sub-step 11213. Similarly, in the coding step 1141, the data part DP is respectively encoded with an outer code, e.g. particular CRC code (i.e. block check sequence), tailed bitten, and encoded with an inner code (e.g. convolution code) in sub-step 11411, 114112, 114113.
Next, the resulting bits in steps 1121 and 1141 are respectively punctured (i.e., deleted) in steps 1122 and 1142 to provide a number of unpunctured header bits (i.e. the coded header field CHF) and unpunctured data bits (i.e. the coded data part CDP). The coded header field CHF and the coded data part CDP both achieve a desired coding rate defined as the ratio of the information bits to the total bits to be transmitted. For example, the transmitter may puncture 1404 coded header/data bits to obtain 612 unpunctured header/data bits. Typically, the coded header field CHF has a lower coding rate than the coded data part CDP since the coded header field CHF contains key information of the coded block CB.
Finally, in step 116, the coded header part CHP and the coded data part CDP undergo a block shaping procedure including reordering, partitioning, and interleaving to provide the coded block CB (i.e. the RLC/MAC block) for transmission.
FIG. 2 is a flow diagram of a conventional decoding scheme used in a conventional receiver, where a reverse procedure of the coded-block formatting procedure in FIG. 1a is performed on a coded block CB′ to recover the original information bits and a bit error probability (BEP) estimation method referred to as Codec method is used to obtain the BEP of the coded block CB′.
As shown, in step 216, the coded block CB′ is de-shaped by a block de-shaping procedure including de-reordering, de-partitioning, and de-interleaving to provide a channel hard output CHO comprising a header hard output HHO and at least one data part DHO respectively corresponding to the coded header part CHP and the coded data part CDP in FIG. 1b. 
Next, in steps 212 and 214, the header hard output HHO and the data hard output DHO are respectively depunctured and decoded to provided a decoded header part DHP and at least one decoded data part DDP respectively corresponding to the header part HP and the data part DP in FIGS. 1a and 1b. 
More specifically, in steps 2122 and 2142, reverse procedures to steps 1122 and 1142 are performed. That is, the header hard output HHO and the at least one data hard output DHO are de-punctured to insert bits in positions where the bits were punctured. In the example in FIGS. 1a and 1b, the receiver de-punctures the 612 bits of the header hard output HHO/data hard output DHO to recover the original 1404 unpunctured header/data bits. Next, in steps 2121 and 2141, reverse procedures to steps 1121, 110 and 1141 are performed to decode the resulted bits in steps 2122 and 2142 into the decoded header part DHP and the at least one decoded data part DDP. The re-encoded header part RHP and the re-encoded data part DDP are designated as re-encoded whole block RWB.
To further determine whether the coded block CB′ is received correctly, steps 112′ and 114′, respectively similar to steps 112 and 114 of FIG. 1a, are performed. In step 112′, the decoded header part DHP is respectively encoded and punctured in steps 1121′ and 1122′, similar to steps 1121 and 1122 of FIG. 1a, to generate a re-encoded header part RHP. Similarly, in step 114′, the decoded data part DDP is respectively encoded and punctured in steps 1141′ and 1142′, similar to steps 1141 and 1142 of FIG. 1a, to generate a re-encoded data part RDP. Next, in step 230, the channel hard output CHO and the re-encoded whole block RWB are compared to generate the BEP of the coded block CB'.
Since the header part contains key information of the coded block, if the decoded header part DHP indicates a CRC error, the receiver will be unaware of the coding and puncture type of the data hard output DHO so that the data hard output DHO cannot be correctly decoded into the decoded data part DDP. Simply deserting the corrupted coded block CB′ and neglecting its contribution to the overall averaged BEP significantly degrades estimation accuracy and fails tot meet the high accuracy requirement for the EDGE systems. The Codec method is thus no longer applicable in EDGE system.
Accordingly, a BEP estimation method with high accuracy applicable to EDGE systems is called for.